Tissue defects involve an absence of healthy tissue in a body area where such tissue would normally be present. For example, a common tissue defect includes aneurysms, in which there is a defect in a blood vessel wall that causes an outpouching of the vessel tissue. Aneurysm may form in disparate locations such as the aorta, iliac arteries, renal arteries, popliteal arteries, splenic arteries, femoral arteries, tibial arteries, and throughout the neurovasculature. Other common tissue defects include arteriovenous fistulas, intestinal fistulas, colonic fistulas, anal fistulas, hernias and traumatic wounds. Aneurysms and other tissue defects may be treated using surgical methods, such as clipping, or endovascular methods, such as flow diversion techniques using a flow diverter (e.g., a covered stent) or coil embolization techniques using coils or a combination of coils and a stent.
In coil embolization techniques, small metallic coils are delivered to the sac of an aneurysm. The coils pack the sac densely to limit blood flow in the sac, thereby inducing clotting of the blood inside the sac and, eventually, healing of the aneurysm. However, such coil embolization techniques can be used only with aneurysms with a narrow neck region to hold the coil in place. Further, such coil embolization techniques suffer from complications, including the risk of recanalization in which blood flow returns to the sac and further swells the sac.
In coil embolization techniques using a combination of coils and a stent, the stent is used as a device that acts as a scaffolding structure to keep the coil inside the aneurysm volume, as shown in FIGS. 1A-B. After the stent is deployed covering the neck of the aneurysm, a delivery microcatheter is passed through a strut (also called an element) of the stent into the aneurysm dome and embolic coils are deployed through the catheter tip inside the aneurysm dome to fill the aneurysm volume.
A significant problem with coil embolization techniques is that during the process of filling the aneurysm volume, the coils or blood clots at the embolization site sometimes herniate into the parent artery if the stent fails to provide adequate scaffolding. Coils or blood clots herniating into the parent artery may escape from the aneurysm volume and travel downstream into the blood vessel and cause a stroke or other life threatening complications.
Another technique for treating aneurysms is with the use of a flow diverting stent. A flow diverter is placed in a blood vessel such that it spans the neck region of an aneurysm, thereby diverting blood flow away from the aneurysm sac. The stagnant blood inside the aneurysm sac may then clot and the aneurysm may heal.
Flow diverters, however, also suffer from complications. Braided devices used in the neurovasculature are bulky and often cannot access distal aneurysms. Use of these devices may also result in incomplete or delayed aneurysm occlusion, which can lead to delayed aneurysm rupture and stroke. In other vascular beds, such as the aorta or arteries of the lower extremities, covered stents are used to treat aneurysms. The most commonly used materials for covered stents include polytetrafluorethylene (PTFE) and polyethylene terephthalate (PET). Both of these materials add substantial bulk, making the stent unsuitable for use in certain vascular beds, such as the neurovasculature. In addition, these materials tend to be impermeable or only semi-permeable. This limits tissue in-growth into the stent covering and leaves a foreign body that is continuously exposed to blood. Because of this, there is a long-term risk of acute thrombosis and stenosis inside the stent. Moreover, because these stents are impermeable to blood flow they will cut-off blood flow to any vessels adjacent to the aneurysm that are covered with the stent. This can lead to ischemia of critical tissues such as the intestine. Further, blood clots formed at the covered stent implanted site may dislodge and cause a heart attack, stroke, or other life threatening complications.
A significant problem with stents, whether used to provide scaffolding to coils in coil embolization techniques or as a covered stent in flow diversion techniques, is their tendency to kink and failing to achieve good wall apposition when placed in torturous vascular beds. Accordingly, there is a need in the art for improved stents that are more kink resistant and achieve improved wall apposition, while simultaneously serving as a good scaffold for coil-based aneurysm treatment techniques.